The invention pertains to a method for the detection and automatic correction of horizontal or vertical errors in image steadiness during continuous image scanning of films, in accordance with the general definition provided by the main claim.
A substantial disadvantage in the reproduction quality of films on television, as compared to images made with electronic cameras, are the visible horizontal and vertical errors in image steadiness. These partly periodic and partly statistical fluctuations in image position can have several causes. Positioning errors in the film camera or in the negative/positive copy machine, or synchronization errors--flutter and wow--in the film scanner cause problems in image steadiness. Also variations in the dimensions of the perforations in the film stock can have negative effects on steadiness. Even if very tight mechanical tolerances are maintained, the sum of all these errors can cause visible errors in image steadiness on a TV monitor.
The causes, the effects, and the elimination of these errors have been discussed in various publications for a long time, for example Frielinghaus "New Investigations on Picture Steadiness of Motion Pictures in Projection", Journal of the SMPTE, January 1986, page 34 et seq.; Zavada "Challenges to the concept of Cancellation", Journal of the SMPTE, December 1981, page 1173 et seq.; and Wright "Image Steadiness in Television Presentation", Journal of the SMPTE, October 1971, page 812 et seq.
The V.K. patent GB No. 165 417 A describes a method for the measurement and electronic correction of errors in image steadiness. The video signals from a sequence of film images are evaluated using a motion vector measurement of the film image content. However, such a measurement is bound to fail if the image does not provide adequate information, for example: sharp, fixed edges. Also the measurement can produce false results when the camera moves or zooms. The method uses two dimensional interpolators to correct image position without affecting the scanning of the film image. The use of interpolators, however, is expensive and degrades the resolution of the film picture by reason of their two-dimensional low-pass filter effect.
In the V.K. patent GB No. 2 145 899 A a further procedure is described in which the edge of the film image is scanned and is compared to the studio synchronization pulse in order to determine the error in image position. The selection of image storage locations is controlled to correct image position and ongoing scanning is not affected. However, since there are film formats which do not provide a clear frame for the film image and in which film images can even overlap in an unfavorable case, for example in Cinemascope or Super 8 formats, inaccuracies in the determination of position errors can be produced by this method. Besides, different film formats provide different frame sizes. Frame size can change continuously in films. The correction of position by storage location selection also requires an image memory having a storage capacity greater than the total content of the images. Furthermore, the resultant selection, with its variable timing signals, complicates distribution of these timed signals in a studio.
A method for correcting synchronization errors in a film scanner is proposed in the European patent EP No. 0 162 486 A2, wherein tachometer pulses are generated by a toothed roller which is coupled to the film for the control of the horizontal driving frequency of a CCD line sensor array. However, a disadvantage of the method is that the control of the horizontal driving frequency causes changes in the integration time of the CCD and, thus, changes the white-level and the black-level for image elements of the video signal. Compensation of that change is very difficult. Furthermore, the integration effect of the toothed roller, and the motion of each tooth relative to the perforation hole, make quick and precise correction for vertical errors impossible. Finally, no horizontal image steadiness correction is provided by this method.